Katainen said that the eurozone countries must do all they can to preserve the common currency, since the other alternatives would be "horrifying". He spoke in Rovaniemi, capital of Finnish Lapland, as his conservative National Coalition Party kicked off its summer congress.
The former finance minister said it was essential for European countries to support Spain's banks so that the Spanish state can survive. He compared the situation to that in May 2010 when the eurozone's temporary crisis fund was established.
"I can only underline this sense of concern," said Katainen. "But we still have a chance to survive if we can just get proper decisions and if everyone sticks to the rules that have been agreed."
The conference caps off a hectic week that saw Finland's largest company announce sweeping job cuts and lose nearly one fifth of its market value, while the EU braced for potentially fateful Greek elections this weekend.
On the bright side, Katainen was this week awarded Germany's prestigious Golden Ludwig Erhard prize after meeting with Chancellor Angela Merkel in Berlin, and also met with his Nordic counterparts in northern Norway. At home, his six-party government sailed through a vote of confidence in Parliament.
The 40-year-old Katainen is to be re-elected to head the party at the three-day meeting. Having led the party since 2004, he is running for the post unopposed.
Last weekend the Centre and Swedish People's parties elected new chairs.